The present invention relates to a belt for use in a Continuously Variable Transmission.
Such a belt is generally known, more in particular from the U.S. Pat. No. 3,720,113. Belts satisfying the described characteristics mentioned in this document are known in practice, and prove to perform satisfactorily. The set of rings shown by such belts will be described as adjacent rings, alternatively denoted hoops or strips, and adjacent meaning directly adjacent, in other words such that mutual abutment may take place between adjacent rings in a set.
Despite the known belt is at present commercially available for more than a decade, and is technically known for even many more years, it is in practice still found that the endurance time of belts may still be significantly improved.
It is therefor an object of the invention to enhance durability of the known belt while maintaining and preferably improving the power transmitting capacity of the known belt. According to the present invention this may in a surprisingly simple manner be achieved by that described below.
A problem underlying the invention is how to determine one ore more specific features of a belt related to the endurance thereof. One such feature is suggested by U.S. Pat. No. 4,579,549, teaching that the innermost ring, alternatively denoted hoop, is subjected to the biggest tensile stress of all the hoops. Decrease of tensile stresses of the innermost hoop is mentioned to enhance the service life thereof and therewith of the belt. For achieving such, the document further indicates the hoops of a set to be xe2x80x9csuperimposed with no clearance between abutting hoopsxe2x80x9d. This indication is in the document explained to mean that each of the hoops has an outer circumferential diameter adapted to be equal to, or larger than, an inner circumferential diameter of an abutting outer hoop. The technical effect of such measure is suggested to be that the tensile stresses of the hoops increase with the increase in diameter thereof. This measure is further explained to be found as an inverse or opposite effect found at prior art belts xe2x80x9cwhich have clearances of 30 xcexcm between abutting hoopsxe2x80x9d, and which prior art belts featured a decreasing tensile stress with an increasing diameter of the respective rings. It is here remarked that said latter prior art feature fits the teaching of above mentioned earlier publication U.S. Pat. No. 3,720,113 in that practically no play could include a play of 30 xcexcm for as far as belts of a dimension within the range for regular automotive application are concerned.
A confirmation of the teaching of above mentioned publication U.S. Pat. No. 4,579,549 is provided by the recent publication of Japanese patent abstract No. 10169719 of application No. 08332187. In this publication the rings of a set are indicated to require a likewise so called minus clearance or, alternatively denoted, a negative play.
Both abstract and U.S. Pat. No. 4,579,549 suggest the characteristic of play between rings of a set to be important in relation to life time of a belt. These documents however provide a measure effecting a confusingly inverse effect on distribution of belt tension in the rings of a set.
As to U.S. Pat. No. 3,720,113, when interpreted with the teaching of U.S. Pat. No. 4,579,549, pointing to play between rings, it is found that rings of the first known belt xe2x80x9care super imposed with practically no play.xe2x80x9d For a practical implementation and for use as a starting point for improvement, this indication is too vague. An obvious interpretation of this teaching however, would be to utilise the inverse expression, indicating that the rings are superimposed with some play, be it practically absent. Belts of this nature are likely to show the distribution of tensile stresses conforming to the distribution shown as prior art in U.S. Pat. No. 4,579,549, i.e. will have highest tensile stress at the innermost ring. Adopting the solution of the latter U.S. Pat. No. 4,579,549 document, it is unsatisfactory found that this solution leads to still one weakest ring, however, now located at the outermost side of a ring set.
The present invention overcomes the drawbacks of all prior art by prescribing a play of nominal value zero. In this way an even distribution of ring stress over at least the majority of the rings of a set is effected, realising a balanced transfer of tensile stress between each relevant ring. Moreover, the level of tensile stress so realised in any ring of a set is significantly lower than that in the ring with highest tensile stress in any of both known solutions. It will be found that with a belt according to the invention the life time of a belt is increased to such extend that durability may with impunity be exchanged against the level of torque to be transmitted, e.g. the effect may be utilised by applying the essentially same belt in environments with higher torque to be transmitted. Should all pairs of rings be accomodated with play of nominal value zero, it will be found that life time is enhanced in respect of belts with any above mentioned prior art measure, not in the last instance because belts are in practice found to fail either through failure of the innermost ring or through failure of the outermost ring.
In order to reproduce the above effect in a sufficiently consistent and reliable manner a favoured embodiment according to the invention requires that the nominal value of zero is realised by a tolerance of 0.00005 times the outer diameter of the inner ring of a relevant pair of rings, plus or minus of the diameter. The technical effect of this measure according to the invention is that the differences of lifetime effect will remain within an accepted bandwidth for technically equivalent belts. In further detail of this effect, it will be found that that no significant difference in life time can be remarked as a consequence of this measure, since the effect hereof is that the spread of life time consequence of this measure will normally remain within the natural life time spread caused by metal fatigue.
Yet another preferred embodiment of the invention requires the mutual play between the innermost pair of adjacent rings is of negative value. It is acknowledged by the invention that the innermost ring of a set, as described by U.S. Pat. No. 4,579,549, xe2x80x9cis subjected to both tensile forces and frictional forcesxe2x80x9d. However, as indicated below, the measure subsequently taught by said publication is rejected. Rather, the favoured embodiment applies the idea underlying the present invention to solve the problem of the first ring, i.e. to take account of the tension each ring is individually subjected to. Combining this idea with the insight underlying the invention that the innermost ring is subjected to certain forces specific thereto, it was found according to this specific aspect of the invention that the tensile stress should be kept relatively low in order to keep the total level of stress in the innermost ring at a level conforming to the level in remainder of the rings of a set. In this respect it was recognised that the frictional force following from interaction between the innermost ring and the transverse elements as mentioned in U.S. Pat. No. 4,579,549, when compared to frictional force resulting from ringxe2x80x94ring interactions is high, so that an additional frictional force should be calculated with for the innermost ring. At the innermost ring it should further be calculated with so-called Herze tensions resulting from element-ring contact. A favoured manner of keeping the level of tension in the innermost ring at the level of the in-between rings of a set is, according to the invention, to reduce the tensile stress by providing some negative play for the innermost ring only.
In a further advancement of above measure directed to the play of the innermost ring specifically, the invention is further characterised in that the outermost diameter of the innermost ring is of a value (1xe2x88x92Z) times the inner diameter of the adjacent ring, Z being of a value smaller than 0.0008. A technical effect of the so specifically identified range is that the reliability and consistency of the effect of the measure related to the innermost belt is increased, while taking into account further sources of tension specific to the innermost belt, including e.g. so called Herze tensions originating from the element-ring contacts. In this respect the value of greater than 0.0001 effects yet a further enhancement. It will be found that, although the parameter Z to some extent varies with the application of a relevant belt, a belt is stressed in conformance with the in-between ring, thus having a conforming life time, when the parameter Z is kept in the above the range.
A further specific aspect of the invention relates to a belt wherein the mutual play of the outermost pair of adjacent rings is of positive value. The technical effect hereof is that compensation is provided for tension sources other than tensile stress, specific to the outer rings, such that the total tension in the outer ring may be brought into conformance with, i.e. will not be higher than, the tension in the further rings of a set. An insight underlying this aspect of the invention is that a tension source specif to the outer rings is caused by subjection to intense contact with transverse elements, i.e. with the boundary opposite to the so called saddle part at the instant where the belt should escape out of the pulley in which it is to be operated. Since the elements are clamped between the sheaves of a pulley, they tend to rotate therewith. For transmitting force to the other puley however, the belt and the elements should cross to the other pulley. Thus the elements are pulled out of the sheaves at the instance of leaving the pulley, thereby causing intense contact between the so-called ear part of a transverse element and the outermost ring. In a further development of this aspect the inner diameter of the outermost ring is of a value (1+Y) times the outermost diameter of the adjacent ring, Y being of a value smaller than 0.0004 and preferably greater than 0.00005. When the values of this specific range are followed the technical effect indicated at the range for the innermost ring will be further enhanced.
A particular aspect of the invention is characterised in that the thickness of one or both of the innermost and the outermost ring of the set is significantly by less than the nominal thickness of in between rings of the set. The invention therefor also relates to a belt for use in a continuously variable transmission, in particular for automotive application, comprising at least one set of nested metal rings, the set interacting with transverse elements provided slidably along the set, and the rings of the set being accommodated with small mutual play between each pair of adjacent rings, whereby the thickness of one or both of the innermost and the outermost ring of the set is significantly less than the nominal thickness of in between rings of the set. With such a measure the invention provides an alternative means of realising an evenly distributed tension level over the rings of a set. The insight utilised in this invention is that a smaller thickness realises a lower bending stress in the relevant ring thereby lowering the tension level in the innermost and/or outermost ring, allowing the relevant ring to receive tension from sources specific therefor, without penalty in regard of life time.
Yet another particular aspect of the invention relates to the measure wherein the material composition of at least one of the innermost and the outermost ring of the set significantly differs from that of the in between rings of the set, such that the elasticity modulus thereof is significantly lower than that of in between positioned rings. The invention therefor also relates to a belt, for use in a continuously variable transmission, in particular for automotive application, comprising at least one set of nested metal rings, the set interacting with transverse elements provided slidably along the set, and the rings of the set being accommodated with small mutual play between each pair of adjacent rings, whereby the elasticity modulus of one or both of the innermost and the outermost ring of the set is significantly less than the nominal thickness of in between rings of the set. With such a measure get another alternative for realising an evenly distributed tension over all rings of a set is realised. The technical effect is comparable to the preceding alternative.
In a further elaboration of the latter aspects the value for thickness and elasticity modulus of said the innermost and the outermost ring is at least twenty percent (20%) less than the average value of the parameters at the in-between rings. When the values of this specific ranges are followed, the technical effect in durability and torque to be transmitted, indicated a.o. at the range for the innermost ring, will be further enhanced.